1. Field of the Invention
The present invention is related to devices used to monitor and control coherent radiation sources. More specifically, it relates to devices that measure and maintain output wavelengths of coherent radiation sources.
2. Description of Related Art
The use of wavelength tunable coherent radiation sources generally requires that the output wavelength of said source be both stable and accurate during extended periods of operation. Current tunable sources, which use tuning methods including MEMS technology to vary the mechanical dimensions of a laser cavity, or variation of the source temperature, may not have the required long-term stability. A means for continuously monitoring and controlling the output wavelength of the source during operation is then required.
The use of Fabry-Perot etalons in the detection and analysis of coherent radiation is well known in the art. One example is the patent to Byer et al. (U.S. Pat. No. 4,172,663), in which a plurality of etalons are constructed with pairs of half-silvered mirrors. The gap between etalon mirrors is filled with a gas which may be adjusted in pressure to correct the optical properties of the etalon. Illumination from a coherent source is directed sequentially through a series of etalons of increasing resolution, producing interference patterns that are sequentially directed onto a spatial detector. The radii of the first and second rings of the respective interference patterns are measured to derive fractional fringe order measurements of successively higher resolution, so that the source wavelength is determined in successively increasing steps of higher resolution.
In another example, the patent by Siebert (U.S. Pat. No. 4,536,089) describes a plurality of etalons of varying thicknesses to detect and determine the wavelength of incident coherent radiation in an environment where incoherent radiation is also present. The etalons utilized in this case are fabricated by deposition of reflective coatings on either side of a glass plate of varying thickness. The source wavelength is determined by comparison of transmitted signal intensities at each etalon thickness.
In a third example, the patent to Yokota (U.S. Pat. No. 4,822,998) comprises a plurality of etalons fabricated by processing a silicon dioxide layer, which is deposited onto a glass substrate, to have a series of stepped thicknesses. A reflective coating is disposed on either side of the silicon dioxide layer. Determination of a coherent source wavelength is made in the same manner as the example given immediately above.
As exemplified by the given examples, existing technology requires precision fabrication techniques, are relatively bulky, and have high manufacturing expense. A need then exists for an improved device to monitor and maintain the wavelength of a coherent radiation source.